The present invention relates to an electrical gaseous discharge device for pumping a laser medium, and more particularly to an electrical gaseous discharge device having an auxiliary electrode or pre-ionizing the gas medium in the vicinity of the main cathode to initiate the plasma discharge inside the gaseous discharge tube.
Various lasers use electrical gaseous discharge as the means of pumping the laser medium in order to achieve population inversion. In order to obtain high laser power output, moderately high discharge gas pressure is used. Typically, for continuous wave operation of a CO.sub.2 laser, the gas pressure is in the region of 25 torr to 750 torr (atmospheric pressure). The TEA (Transverse Exited Atmospheric), and Excimer lasers operate at about one to ten atmospheric pressures. Many of the solid state lasers such as YAG, glass, GGG and Alexandrite lasers are pumped by the photons generated from Krypton or Xenon flash lamps filled to a pressure of one to five atmospheres.
One of the problems encountered with high pressure gaseous discharge is the initialization of the discharge itself. In order to start up a discharge, i.e., cause an electrical current to flow from one electrode (anode) to the other (cathode) through the gas medium, there must be present in the gas medium enough electrons to carry the current. Under normal (room) conditions of temperature and cosmic radiation, only an extremely small quantity of electrons is present in the gas medium. As a result, a voltage as high as two to five times the normal operating value is required to apply between the anode and cathode in order to provide an electric field high enough to break down the gas molecules and hence increase the electron density between the electrodes. As the electron density increases, the ionization rate of the gas molecules also increases, eventually leading to a self-sustained discharge. Since the value of the high striking voltage necessary to initiate the discharge is not a well defined one, such method leads to inconsistency in discharge plasma initialization. The problem becomes more and more serious as the gas pressure and discharge length are increased.
Another drawback of using high striking voltage to initiate a plasma discharge is the unavoidable high current spike that immediately follows at the on-set of the discharge. This high current spike may not be a desirable feature as it may lead to the following effects:
(a) Degradation of the discharge electrodes due to the high electric field. PA0 (b) Degradation of the discharge gas due to chemical dissociation of the molecules. PA0 (c) The laser beam will exhibit gain-switching effect, producing an initial high beam power spike with poor spatial mode profile.
Several techniques have been used conventionally in order to pre-ionize the gas medium before the main discharge voltage is applied. One of these techniques involved the generation of a small volume of discharge plasma near the main discharge anode electrode. This requires an addition of a high voltage power supply, and hence added cost. Both ac and dc power supply had been used for this purpose.
Other techniques that had been used for pre-ionization relied on the ionization effect of external radiations such as ultra-violet (UV) light, x-ray and electron beam. Again, additional equipment is needed to serve the purpose.
Accordingly, one of the objects of the present invention is to provide a gaseous discharge device capable of generating a high electric field over a small region inside a gaseous d.c. discharge tube near the main discharge cathode in order to facilitate the initialization of the main discharge.
Another object is to provide an electrical circuit such that the main plasma discharge can be started without the undesirable high current spike.
Still another object of the present invention is to provide a gaseous discharge device of making use of the main discharge d.c. power supply for the generation of such a small region of discharge so that no additional power supply is required.
A further object of the present invention is to provide a gaseous discharge device capable of generating the main d.c. discharge with an initialization discharge region whose intensity increases with increase in the main plasama discharge current for better main discharge stability.
Still a further object of the present invention is to provide a gaseous discharge device capable of a better pre-ionization of the gas so as to ensure a better consistency in discharge characteristics from one discharge pulse to the other when the discharge is operating in a pulsing mode.